The present invention relates to self-centering workpiece holding and positioning fixtures for machine tools such as lathes.
The invention has particular reference to workpiece holding and positioning fixtures of the kind comprising a body attachable to a rotary spindle of a machine tool and having a pair of jaws rotatable about a common axis which intersects the axis of rotation of the said body, a first servo control for moving one of the said jaws along the said common axis, a second servo control controlling a clutch for connecting the other jaw to the body of the fixture, and a third servo control for rotating the said other jaw about said common axis via a transmission including a sprocket which is coaxial with said other jaw.
In one self-centering fixture of this kind, herein referred to as "a self-centering fixture of the kind specified", which is disclosed by the U.S. Pat. No. 3,790,181 the clutch consists of a ring gear formed on the surface of the said other jaw and cooperating with a conjugate ring gear formed upon the body of the chuck.
The second servo control which controls this clutch comprises a piston fixed to a shaft constituting an extension of the said other jaw and slidable within a cylinder in the body of the fixture to form a double-acting hydraulic actuator. When it is necessary to cause rotation of the two jaws and of the workpiece gripped between them through a predetermined angle, for example 90.degree., the said hydraulic actuator is operated to move the said other jaw axially by an amount sufficient to effect disengagement of the two ring gears which constitute the clutch. Disengagement having been effected, the third servo control then comes into operation: this servo control may comprise a rotary hydraulic motor which acts upon the said other jaw by way of a transmission which includes a worm wheel keyed on to the said shaft of the said jaw, and engaged by a worm screw driven by the said hydraulic motor. After rotation of the said other jaw, and with it the workpiece and the first jaw, the actuator which controls the clutch comes into action again to effect axial movement in the opposite direction of the said other jaw, re-engaging the clutch by bringing the ring gears formed respectively on the said other jaw and on the body into engagement with each other.
A disadvantage of the aforesaid known system is that it is necessary to displace the workpiece, which may weigh several hundredweight, laterally in relation to the axis of rotation of the fixture in order to effect engagement and disengagement of the clutch. It is consequently necessary to stop the operation of the machine tool on which the fixture is mounted for the period of time necessary for causing rotation of the jaws around the common axis of the jaws, while ensuring that the jaws continue to grip the workpiece, in order to present for machining a surface of the workpiece which is angularly displaced from the surface previously machined. Another disadvantage lies in the need to keep the workpiece gripped between the jaws during the said lateral displacements of the jaws relative to the axis of rotation of the body of the fixture.